In a lead-acid storage battery, positive and negative battery posts are provided and normally these posts protrude through the battery cover wall. These posts are electrically connected below the cover to a lead or lead alloy grid form containing active battery paste material and this construction normally results in a rigid structure. Some form of liquid-tight seal is usually provided between the cover and terminal post to preclude leakage of electrolyte. Due to gradual electrode plate enlargement, commonly referred to as "grid growth," during battery cycle life, the plates produce vertical forces on the electrode plate-terminal structure. These internal forces cause the terminal post to be pushed upward thereby destroying the post-to-cover seal and permitting electrolyte leakage through the broken seal if the cover and container are an integral unit, such as is the case in covers heat sealed to the containers. In batteries utilizing individual cell covers, the post is normally fused light-tight to the cover bushing and the cover is sealed to the container by asphaltic compound materials or other such flexible materials. When grid growth occurs in this configuration, the entire cover is moved vertically and will result in the cover becoming tilted and may result in electrolyte leakage through possible breakage of the cover-container seal. Considerable effort has been made to develop a reliable post-to-cover seal; however, due to various reasons, the reliability of the prior embodiments has not been satisfactory.
In the battery terminal post seal illustrated in U.S. Pat. No. 1,094,173 of James M. Skinner, spaced convolutions around each terminal post present a series of steps interlocking with the sealing compound extending between them thereby offering resistance to vertical strains tending to separate the terminals and sealing compound in which they are embedded and opposing resistance to transverse strains tending to displace the terminals with respect to the sealing compound. Failure of the seal may occur when the post moves in the vertical direction, thereby forming a separation in the seal material and permitting electrolyte leakage.
Another form of battery terminal post seal is illustrated in U.S. Pat. No. 1,434,307 of Edgar A. Miller. Sealing material is deposited in an outwardly opening well surrounding the terminal post. Such a seal has the serious disadvantage of causing the failure of the seal between the sealing material and the case wall as a result of an outward movement of the battery post due to the arrangement of the frustoconical surfaces involved.
Another form of battery terminal seal is illustrated in U.S. Pat. No. 1,460,895 of Leon A. Doughty et al. As shown therein, the post is provided with interlock ribs embedded in the sealing material. Outward movement of the terminal post in this structure may cause a movement of the post relative to the sealing material as a result of rupture of the sealing material by the ribs moving through the relatively large surrounding body.
Another form of terminal post seal is illustrated in U.S. Pat. No. 3,918,993 of Edward Adderley et al. As shown therein, the sealing material is provided in a central bore of the post having radial passages conducting the sealing material outwardly to the surrounding wall of the battery case.
Another form of battery terminal post seal is illustrated in U.S. Pat. No. 4,224,388 of Steven J. Stadnick. As shown therein, the terminal post is provided with a lower ceramic washer which acts like a piston to compress the sealant when a nut on the terminal post is tightened. Double washers are employed to maintain a minimum tension on the post.
A vexatious problem occurs in the prior art terminal post seals in that movement of the terminal post outwardly through the sealing material may expose a corroded portion of the terminal post to the sealing material thus reducing the sealing effect and, in a number of instances, causing leakage of the battery.